1. Field of the Invention
The present invention relates to a projection lens for a projection television, and more particularly relates to a wide angle type projection lens with a favorable optical performance, which is compact and low in cost, and which can be used for a very bright projection television with a value of F.sub.NO less than or equal to 1.15, or in the extreme case less than or equal to 1.1.
2. Description of the Related Art
In recent years the demand has increased for projection television systems, which are nowadays used not only for analog display of broadcast television images but also as analog displays for electronic image replaying mechanisms such as video tape recorders, video disk players, and the like. Projection televisions are also being used for display of digitalized picture data, in both character and graphical format, such as computer output and video text data. And recently large screen projection type televisions are becoming more and more popular even for general household use. Accordingly enhancements of brightness, contrast, and resolution for projection televisions are being strongly pursued, in order to bring their picture quality closer to that of normal direct vision type televisions, and concurrently pressures for compactness and cost reduction are very strong.
Further, the picture quality of a projection television is dictated by the performance of the projection lens used, and the projection lens is required to be capable of forming clear images not only around the optical axis but also in regions remote from the optical axis. From the view point of reducing the size and cost of the projection television, it is important to reduce the size and cost of the projection lens which is a major factor in determining the size and cost of the projection television.
In the prior art relating to projection lenses for projection televisions, although in a compound type projection lens there were combined together a large number of glass lenses, these glass lenses were in general spherical lenses, and accordingly the number of lenses in the combination became undesirably large, which entailed undue bulk and cost; and further, due to the specific gravity of glass being relatively high, the compound projection lens as a whole was rather heavy.
Recently, however, it has become possible to produce large aperture aspherical type plastic lenses with relatively high accuracy, and accordingly various compound projection lenses have been contemplated incorporating combinations of glass lenses and plastic lenses, whereby it is possible to maintain the optical performance at a high level while reducing bulk and cost.
Plastic lenses are generally manufactured by a process of injection molding using a metal mold, and for forming a high accuracy plastic lens as a matter of course a high accuracy metal mold must be used. Thus, although the introduction of such compound lens has contributed to the reduction in the cost of projection lenses through reduction in the number of lens elements used in each lens system and other factors, the high cost of the metallic molds cannot be justified in some applications.
In fact, with a currently manufactured mainstream type compound projection lens of the above described hybrid type in which both plastic and glass lenses are used, apart from the so called back lens which is the one of the lenses which is farthest from the screen (closest to the face plate of the CRT) and which is typically of negative power and presents a strongly concave surface towards the direction of the screen (in the direction away from the face plate of the CRT), typically four lenses are used, of which one is a glass lens which is of strongly positive power, and the other three are individually and independently formed plastic lenses. Accordingly, for the manufacture of such a compound projection lens, it is required initially to provide three different metal molds for making these three individual plastic lenses, which entails substantial initial cost.
Further, another problem with the prior art has been that, since the coefficient of linear expansion of plastic material is remarkably large as compared with that of optical glass and also the rate of change of refractive index of plastic material with change of temperature is also large, therefore the change of back focus for a plastic lens due to change of temperature is relatively great, and therefore its performance for image formation is deteriorated. With a projection television, this has caused the problem of picture quality deterioration. And, in view of the requirements for provision of an acceptable optical performance relating to geometrical optical aberration and the like and furthermore for wide angle performance and compactness, in the prior art there have been very few compound projection lenses for projection televisions which achieved a value of F.sub.NO less than or equal to 1.15.